Heat milk in microwave for 2 minutes on HIGH.
Add the chocolate chips and allow it to sit for about 2 minutes.
Stir until chocolate is blended. If necessary, heat at 10 second increments and stir again.
In medium bowl, mix the sugar and egg YOLKS together.
In a small bowl, whisk together the cocoa, cornstarch and salt together. Then, blend it into the egg/sugar mixture until well incorporated.
Next, add in the chocolate milk mixture until well blended then pour all contents into a medium saucepan.
Over MEDUIM heat, stirring constantly, until thickened and just beginning to bubble. Consistency should hold its form like pudding.
Remove from the heat and stir in the butter and vanilla until well blended. Let it rest for about 20 mnutes until it becomes "warm", stirring every 5 minutes to maintain its smoot texture.
Pour into cooled pie shell and let it sit on cooling rack until it has completely cooled.
Place in a pie container or cover with plastic wrap in the fridge. Chill for a few hours before slicing.